The present invention relates to a method for the electrochemical oxidation of semiconductors.
There has heretofore been known a wet anodization method as one of techniques for providing porosity to a semiconductor or for forming an oxide film on the surface of a semiconductor. The techniques for forming an oxide film on the surface of a semiconductor also include an electrochemical oxidation method utilizing an electrochemical reaction. In late years, there has been proposed a field emission-type electron source prepared by a process using a wet anodization method and an electrochemical oxidation method.
For example, as shown in FIG. 20, this kind of field emission-type electron source 10 (hereinafter referred to as xe2x80x9celectron source 10xe2x80x9d for brevity) comprises an n-type silicon substrate 1 as a conductive substrate, and a strong-field drift layer 6 (hereinafter referred to as xe2x80x9cdrift layer 6xe2x80x9d for brevity) which is composed of an oxidized porous polycrystalline silicon layer and formed on the side of one of the principal surfaces of the n-type silicon substrate 1. Further, a surface electrode 7 composed of a metal thin film (e.g. gold thin film) is formed on the drift layer 6, and an ohmic electrode 2 is formed on the back surface of the n-type silicon substrate 1. In this structure, the n-type silicon substrate 1 and the ohmic electrode 2 serve as a lower electrode 12. While the electron source 10 illustrated in FIG. 20 includes a non-doped polycrystalline silicon layer 3 interposed between the n-type silicon substrate 1 and the drift layer 6, there has also been proposed another electron source designed such that the drift layer 6 is formed directly on the principal surface of the n-type silicon substrate 1.
In an operation of emitting electrons from the electron source 10 illustrated in FIG. 20, a collector electrode 21 is disposed in opposed relation to the surface electrode 7. Then, after a vacuum is formed in the space between the surface electrode 7 and the collector electrode 21, a DC voltage Vps is applied between the surface electrode 7 and the lower electrode 12 in such a manner that the surface electrode 7 has a higher potential than that of the lower electrode 12. Simultaneously, a DC voltage Vc is applied between the collector electrode 21 and the surface electrode 7 in such a manner that the collector electrode 21 has a higher potential than that of the surface electrode 7. Each of the DC voltages Vps, Vc can be appropriately arranged to allow electrons injected from the lower electrode 12 into the drift layer 6 to be emitted through the surface electrode 7 after drifting in the drift layer 6 (the one-dot chain lines in FIG. 20 indicate the flow of the electrons exe2x88x92 emitted through the surface electrode 7.). The surface electrode 7 is made of a metal material having a small work function.
While the electron source 10 illustrated in FIG. 20 has the lower electrode 12 composed of the n-type silicon substrate 1 and the ohmic electrode 2, there has also been proposed another electron source 10 as shown in FIG. 21, in which a lower electrode 12 made of a metal material is formed on one of the principal surfaces of an insulative substrate 11. The electron source 10 illustrated in FIG. 21 emits electrons in the same process as that of the electron source 10 illustrated in FIG. 20.
Generally, in this kind of electron source 10, a current flowing between the surface electrode 7 and the lower electrode 12 is referred to as xe2x80x9cdiode current Ipsxe2x80x9d, and a current flowing between the collector electrode 21 and the surface electrode 7 is referred to as xe2x80x9cemission current (emitted electron current) Iexe2x80x9d. In the electron sources 10, an electrode emission efficiency (=(Ie/Ips)xc3x97100[%]) becomes higher as the ratio (Ie/Ips) of the emission current Ie to the diode current Ie is increased. In this connection, the emission current Ie becomes higher as the DC voltage Vps is increased. This electron source 10 exhibits electron emission characteristics having a low dependence on the degree of vacuum, and can stably emit electrons at a high electron emission efficiency without occurrence of a so-called popping phenomenon.
If the electron source 10 illustrated in FIG. 21 is applied as an electron source of a display, the display may be configured as shown in FIG. 22. The display illustrated in FIG. 22 comprises an electron source 10, and a faceplate 30 which is composed of a flat-plate-shaped glass substrate and disposed in opposed relation to the electron source 10. A collector electrode (hereinafter referred to as xe2x80x9canode electrodexe2x80x9d) 21 composed of a transparent conductive film (e.g. ITO film) is formed on the surface of the faceplate 30 opposed to the electron source 10. The surface of the anode electrode 21 opposed to the electron source 10 is provided with fluorescent materials formed in each of pixels, and black stripes made of a black material and formed between the fluorescent materials. Each of the fluorescent materials applied on the surface of the anode electrode 21 opposed to the electron source 10 can generate a visible light in response to electron beams emitted from the electron source 10. The electrons emitted from the electron source 10 are accelerated by a voltage applied to the anode electrode 21, and the highly energized electrons come into collision with the fluorescent materials. Three type of fluorescent materials having luminescent colors of R (red), G (green) and B (blue) are used as the fluorescent materials. The faceplate 30 is spaced apart from the electron source 10 by a rectangular frame (not shown), and an sealed space formed between the faceplate 30 and the electron source 10 is kept in vacuum.
The electron source 10 illustrated in FIG. 22 comprises an insulative substrate 11 composed of a glass substrate, a plurality of lower electrodes 12 arranged in lines on the surface of the insulative substrate 11, a plurality of polycrystalline silicon layers 3 each of which is formed on the corresponding lower electrode 12 in a superimposed manner, a plurality of drift layers 6 each of which is composed of an oxidized porous polycrystalline silicon layer and formed on the corresponding polycrystalline silicon layer 3 in a superimposed manner, a plurality of isolation layers 16 each of which is composed of a polycrystalline silicon layer and embedded between the adjacent drift layers 6, and a plurality of surface electrodes 7 which are formed on the drift layers 6 and the isolation layers 16, and arranged in lines to extend in the crosswise direction of the lower electrodes 12 so as to cut across the drift layers 6 and the isolation layers 16.
In the electron source 10, the drift layers 6 are partly sandwiched between the corresponding lower electrodes 12 arranged on the surface of the insulative substrate 11 and the corresponding surface electrodes 7 arranged in the crosswise direction of the lower electrodes 12, at the regions of the drift layers 6 corresponding to the intersecting points between the corresponding lower electrodes 12 and the corresponding surface electrodes 7. Thus, a certain voltage can be applied between appropriately selected one of the plural pairs of the surface electrode 7 and the lower electrode 12, to allow a strong electric field to act on the region of the drift layer 6 corresponding to the intersecting point between the selected surface electrode 7 and lower electrode 12 so as to emit electrons from the region. This configuration is equivalent to an electron source in which a plurality of electron source elements 10a, each of which comprises the lower electrode 12, the polycrystalline silicon layer 3 on the lower electrode 12, the drift layer 6 on the polycrystalline layer 3, and the surface electrode 7 on the drift layer 6, are arranged, respectively, at the lattice points of a matrix (lattice) formed by a group of the surface electrodes 7 and a group of the lower electrodes 12a. One of the pairs of the surface electrode 7 and the lower electrode 12 to be applied with a certain voltage, can be selected to allow electrons to be emitted from desired one of the electron source elements 10a. 
In a conventional production process for the electron source 10, the drift layer 6 is formed through a film-forming step of forming a non-doped polycrystalline silicon layer on the side of one of the surfaces of the lower electrode 12, an anodization step of anodizing the polycrystailine silicon layer to form a porous polycrystalline silicon layer containing polycrystal line silicon grains and nanometer-order silicon microcrystals, and an oxidation step of rapidly heating and oxidizing the porous polycrystalline silicon layer through a rapid heating method to form silicon oxide films on the surfaces of the grains and the nanometer-order silicon microcrystals, respectively.
In the anodization step, a mixture prepared by mixing an aqueous solution of hydrogen fluoride with ethanol at the ratio of about 1:1 is used as an electrolytic solution. In the oxidation step, a substrate is oxidized by increasing the substrate from room temperature up to 900xc2x0 C. in a short period of time under a dry oxygen atmosphere, and maintaining the substrate temperature at 900xc2x0 C. for 1 hour, for example, using a lamp annealing apparatus. Then, the substrate temperature is reduced down to room temperature.
For example, a conventional anodization apparatus as shown in FIG. 24A is used in the anodization step. This anodization apparatus comprises a processing both 31 containing a electrolytic solution A consisting a mixture of ethanol and an aqueous solution of hydrogen fluoride, and a cathode 33 composed of a grid-like platinum electrode and immersed into the electrolytic solution A in the processing bath 31. An object 30 having a polycrystalline silicon layer formed on the lower electrode 12 is immersed into the electrolytic solution A, and the lower electrode 12 is used as an anode. This anodization apparatus includes a current source 32 for supplying a current between the lower electrode 12 serving as an anode and the cathode 33 in such a manner that the anode has a higher potential than that of the cathode. The anodization apparatus also includes a light source (not shown) composed of a tungsten lamp for irradiating the principal surface of the object 30 (or the front surface of the polycrystalline silicon layer) with light.
A constant current is supplied between the anode and the cathode 33 through an anodization method using the above anodization apparatus to provide porosity from the surface of a target region E in the polycrystalline silicon layer toward the depth direction thereof, so as to form a porous polycrystalline silicon layer containing polycrystalline silicon grains and nanometer-order silicon microcrystals in the target region.
As shown in FIG. 25, the electron source 10 illustrated in FIG. 22 may be produced by arranging a plurality of lower electrodes 12 in lines on the side of one of the principal surfaces of an insulative substrate 11, forming a polycrystalline silicon layer 3 on the side of the above principal surface of the insulative substrate 11, and anodizing the respective regions of the polycrystalline silicon layer 3 superimposed on the lower electrodes 12. In this process, a certain current is supplied to the lower electrode 12 through a current-feeding wiring 12a continuously extending in integral with the lower electrode 12.
In the oxidation step, the porous polycrystalline silicon layer is rapidly heated and oxidized through the rapid heating method, as described above. Differently from this method, a technique using an electrochemical oxidation method of electrochemically oxidizing the porous polycrystalline silicon layer within an electrolytic solution (electrolyte solution) consisting of an aqueous solution of sulfuric acid, nitric acid or the like, in the oxidation step is proposed to form a silicon oxide film having an excellent film quality on all of the surfaces of the silicon microcrystals and the grains. More specifically, in the drift layer 6, when the porous polycrystalline silicon layer is oxidized, a thin silicon oxide layer would be formed on each of the surfaces of a number of silicon microcrystals and a number of grains contained in the porous polycrystalline silicon layer. In view of this point, the proposed electrochemical oxidation method is intended to form a silicon oxide film having an excellent film quality on all of the surfaces of the silicon microcrystals and the grains, by electrochemically oxidizing the porous polycrystalline silicon layer within an electrolytic solution consisting, for example, of 1 mol/l of aqueous solution of sulfuric acid, nitric acid or the like, in the step of forming the drift layer 6.
The porous polycrystalline is electrochemically oxidized using an electrochemical oxidation apparatus of FIGS. 23A and 23B, in which the electrolytic solution A in the anodization apparatus of FIGS. 24A and 24B is replaced with an electrolytic solution B consisting, for example, of an aqueous solution of sulfuric acid. As shown in FIG. 23B, a cathode is set to have the same outside dimension as that of the target region E of the polycrystalline silicon layer. With this electrochemical oxidation apparatus, a certain current can be supplied from a current source 32 between the anode and the cathode 33 so as to electrochemically oxidize the polycrystalline silicon layer in the target region E to form a silicon oxide films on each of the surfaces of the silicon microcrystals and the grains.
In the step of forming the porous polycrystalline silicon layer, the anodization treatment is completed after a certain current is supplied between the anode and the cathode 33 just for a predetermined period of time. By contrast, in the step of electrochemically oxidizing the porous polycrystalline silicon layer, a certain current is supplied between the anode and the cathode 33, and the current supply is terminated at the time when the voltage between the anode and the cathode 33 is increased up to a predetermined value arranged depending on the characteristics (e.g. emission current or withstand voltage) of the electron source 10 (see, for example, Japanese Patent Laid-Open Publication No. 2001-155622)
As compared to the method of rapidly heating and oxidizing the porous polycrystalline silicon layer to form the drift layer 6, the electrochemical oxidation method allows the porous polycrystalline silicon layer to be oxidized under a lowered process temperature. Thus, the restrictions on the material of the substrate can be reduced to facilitate increase in the area and reduction in the cost of the electron source 10.
On the other hand, the conventional electron source 10 produced using the aforementioned electrochemical oxidation method involves a problem of increased variation of the emission current Ie and/or withstand voltage in the surface thereof, and resultingly deteriorated process yield. That is, an electronic device produced using the aforementioned electrochemical oxidation method has a problem of wide variation in their characteristics, such as emission current or withstand voltage.
The characteristics, such as emission current or withstand voltage, would be widely varied due to the following factors:
1) In the aforementioned electrochemical oxidation method, a voltage increment due to the resistance of the electrolytic solution B is included in the voltage between the anode and the cathode. Thus, a voltage incitement due to the formation of the oxide films will be varied according to the variation in the voltage increment caused by the variation in the resistance of the electrolytic solution B.
2) As shown in FIG. 23B, the cathode 33 is set to have the same outside dimension as that of the target region. Thus, a current flows through the electrolytic solution B by paths as shown by the arrows in FIG. 23A, and the peripheral portion of the target region E has a higher current density than that in the remaining region thereof.
3) During electrochemical oxidation, air bubbles are formed on the principal surfaces of the porous polycrystalline silicon layer which is a semiconductor layer to suppress the reaction in the region having the air bubbles formed thereon.
The factor 1) leads to increased variation in the characteristics, such as emission current or withstand voltage, mainly in each of processing batches. The factor 2) or 3) leads to increased in-plane variation in the characteristics, such as emission current or withstand voltage, mainly in a sample, and deteriorated process yield of electronic devices.
In view the above problems, it is therefore an object of the present invention to provide an electrochemical oxidation method capable of reducing the variation in characteristics, such as emission current or withstand voltage, as compared to conventional methods.
In order to achieve the above object, the present invention provides a method for the electrochemical oxidation of a semiconductor layer, wherein an electrode provided on the opposite side of the principal surface of the semiconductor layer is used as an anode, and a current is supplied between the anode and a cathode while allowing the semiconductor layer and the cathode to be in contact with an electrolytic solution, to oxidize the semiconductor layer. In this electrochemical oxidation method, a current is first supplied between the anode and the cathode to initiate the oxidation. Then, the oxidation is terminating under the condition that a corrected voltage value Vt determined by correcting a voltage V between the anode and the cathode in accordance with a voltage inclement V0 based on a pre-detected resistance of the electrolytic solution is equal to a predetermined upper voltage value V1.
According to the above electrochemical oxidation method, the variation in the increment of the voltage between the anode and the cathode in the period between the initiation and termination of the oxidation can be reduced irrespective of the resistance of the electrolytic solution. Thus, the variation of the voltage increment caused by the formation of the oxide films can be reduced to allow the characteristics of an electronic device to have desirably suppressed variation.
In the electrochemical oxidation method, a current density in the principal surface of the semiconductor layer may be controlled in such a manner that the current density in the periphery of the oxidation target region of the semiconductor layer is restrained in increasing to be greater than the remaining oxidation target region. In this case, the in-plane variation of the current density in the oxidation target region can be reduced as compared to the conventional methods to allow the characteristics of an electronic device to have desirably suppressed in-plane variation.
Further, air bubbles formed on the principal surface of the semiconductor layer during the supply of the current may be released from the principal surface while supplying the current. In this case, the oxidation target region can avoid the deterioration in a required reaction therein due to air bubbles to allow the characteristics of an electronic device to have desirably suppressed in-plane variation.